1. Field of the Invention
The present invention relates to an arc control apparatus which controls the deflection of a DC arc caused by the influence of an electromagnetic force generated by a feeder circuit of a DC arc furnace, thereby melting an object in the furnace by evenly discharging the DC arc to the object.
2. Description of the Related Art
A large-current DC arc furnace usually suffers from the problem of the deflection of a heating arc radiated to an object to be melted. The deflection of the DC arc is attributable to the magnetic field generated by a feeder circuit which supplies electric power to the electrode for generating DC arcs and an electromagnetic force generated by the current of the DC arc itself. A deflected DC arc prevents uniform melting of the object in the DC arc furnace. Specifically, the deflected arc locally increases the thermal load on the wall of the furnace during the flat bath phase, leading to deterioration in electric power and refractory materials. This is obvious from the description given on page 28 of industrial Heating "Arc Phenomenon in DC Arc Furnace (4)" written by Nanjo and Yoshida.
Hence, various corrective measures have been proposed to solve the arc deflection problem. FIG. 24 shows the configuration of the feeder circuit in a conventional DC arc furnace which has been disclosed, for example, in Japanese Utility Model Laid-Open No. 2-24290. As illustrated, in the feeder circuit, a smoothing reactor 4 is connected to the negative electrode (-) of the DC output terminal of a thyristor converter 2, which converts the AC power supplied from a furnace transformer 1 into DC power, via a feeding conductor 3. The smoothing reactor 4 is connected to a movable electrode 6 which has a distal end thereof projecting into a DC arc furnace 5.
The positive electrode (+) of the thyristor converter 2 is connected to a furnace bottom electrode 7 of the DC arc furnace 5 through a feeding conductor 3'. The feeding conductor 3' is bent near the furnace bottom electrode 7 in the direction in which the DC arc is deflected by the electromagnetic force generated by the magnetic field created by the feeder circuit to the right, i.e. away from the furnace core in the figure, then it is bent vertically further away from the furnace before it is bent back toward the furnace core to be connected to the positive electrode (+) of the DC output terminal of the thyristor converter 2. The movable electrode 6 is supported by a holder arm 10 that is operable to move up and down.
In the feeder circuit comprising the feeding conductors 3, 3' and circuit elements 4, 2, feeding circuit F-G-A-B-C-D comprises section F-G connecting the holder arm 10 and the negative electrode (-) of the thyristor converter 2, loop A-D in loop A-E connecting the positive electrode (+) of the thyristor converter 2 through the feeding conductor 3' and the furnace bottom electrode 7. In loop A-E, section D-E serves as feeder circuit D-E.
The operation of a device illustrated in FIG. 24 is described below. The moment the DC power from the thyristor converter 2 is supplied between the movable electrode 6 and the furnace bottom electrode 7, whereby current flows in the direction of arrow Y to generate magnetic field B1. In feeder circuit D-E, currents flow in the direction of arrow X, i.e. in the opposite direction from feeder circuit F-G-A-B-C-D, generating magnetic field B2. In this case, the magnetic field B2 produced by feeder circuit D-E is in the direction opposite from magnetic field B1 generated by feeder circuit F-G-A-B-C-D; therefore, magnetic field B2 works in the direction for weakening magnetic field B1. Thus the electromagnetic force applied to the DC arc is weakened by feeder circuit F-G-A-B-C-D and the deflection of the DC arc toward the furnace wall is decreased.
Since the feeder circuitry of the conventional DC arc furnace is configured as described above, magnetic field B1 produced by feeder circuit F-G-A-B-C-D is reduced by magnetic field B2 created by feeder circuit D-E which is arranged horizontally. Magnetic field B1, however, cannot be fully cancelled and the electromagnetic force, although small, remains which and works to cause the DC arc to shift horizontally from the vicinity of the arc generating point to the outside of the feeder circuit.
The magnitude of the residual electromagnetic force is not negligible in a large-current arc furnace of about 100 KA. However, another problem associated with the above described devices is that the discharging direction cannot be controlled at will with respect to a scrap, an object 11 to be melted, preventing quick, efficient and uniform melting of the object 11.